As for a video camera using solid state image sensors in an image pickup part, there are a multi plate type using a plurality of solid state image sensors and a single plate type using a solid state intake sensor. In the video camera of single plate type, color separation filters in mosaic form are mounted on the solid state image sensor, and color signals for each color separation filter obtained from the solid state image sensor are processed whereby brightness signals and color difference signals are formed.
In such a video camera of single plate type using a solid state image sensor, however, since sampling density of each color signal obtained from the solid state image sensor becomes low in comparison with that for multi plate type, a false signal caused by high frequency components shifting to the low range, that is, moire becomes greater and therefore the picture quality of reproduced images is degraded.
As a method in the prior art to suppress moire occurring on brightness signals, for example, the method disclosed in Japanese patent application laid-open No. 170692/1990 is well known, where circuits being different in signal gain for each color signal to respective color separation filters are provided, and the signal gain of respective circuits is set so that the signal amounts of respective color signals become equal for achromatic objects. That is, as the signal amount of each color signal becomes equal, occurrence of frequency components due to the arrangement of the color separation filters can be prevented. As a consequence, moire occurring in brightness signals can be suppressed.
But, if signal gains of a circuit differ for each color signal obtained by color separation filters as in the prior art as above described, a problem exists in that since combinations of color signals on the signals outputted from the solid state image sensors differ between the horizontal scanning periods of nth line and (n+1)th line, the difference of the ratio of R(red), G(green), B(blue) constituting the brightness signals appears between the horizontal scanning periods of the nth line and the (n+1)th line, and the brightness level varies for each horizontal scanning period in the case of image pickup of the chromatic objects.
For example, in the CCD image sensor of picture element mixing type where two upper and lower picture elements are mixed and read out, using a color separation filter shown in FIG. 2 as a color separation filter, if the ratio of R, G and B is made R:G:B=3:3:1 and signal gains for respective circuits are set so that signal amounts of four kinds of Color signals G+Cy, Mg+Ye, Mg+Cy and G+Ye corresponding to each color separation filter become equal to achromatic objects, signal gains for respective color signals are 10/7, 1, 10/8 and 10/9. Then levels of brightness signals for a horizontal scanning period composed of two kinds of color signals G+Cy and Mg+Ye arranged in dot sequential order, become approximately 2R+3.866+2.43B, and levels of brightness signals for a next horizontal scanning period composed of color signals Mg+Vy and G+Ye arranged in dot sequential order, become approximately 2.36R+3.47G+2.5B, and levels of brightness signals for every horizontal scanning period change to chromatic objects (hereinafter referred to as "line pair").